In a wood type golf club head represented by a driver or fairway wood, the golf club head has been formed to have a hollow structure to increase its volume in order to enlarge the so-called sweet spot. In addition, a hollow golf club head fixed with a viscoelastic body on a part of its circumferential wall, e.g., in a sole portion, to improve the hitting impression or adjust the hitting sound on impact has been prevailed. When the viscoelastic body is fixed, the vibration on impact is absorbed by the viscoelastic body to improve the hitting impression and decrease the hitting sound that is offensive to the player's ear.
As a fixing structure to fix a viscoelastic body to a hollow golf club head, for example, a structure disclosed in Japanese Patent Laid-Open No. 2005-160947 has been proposed. In Japanese Patent Laid-Open No. 2005-160947, the following fixing structure is disclosed. A recess portion is formed in a part of the circumferential wall of a golf club head and a viscoelastic body is inserted in the recess portion. The inner circumferential wall of the recess portion forms a screw hole. When a threaded portion is threadably engaged with the screw hole, the viscoelastic body is sandwiched between the distal end of the threaded portion and the bottom portion of the recess portion.
In order to more effectively damp vibration on impact by the viscoelastic body, the viscoelastic body is desirably as close to the circumferential wall of the golf club head as possible. When the viscoelastic body is fixed in the sole portion, the viscoelastic body is desirably fixed to the lowest position to lower the center of gravity of the golf club head. In the fixing structure disclosed in Japanese Patent Laid-Open No. 2005-160947, a viscoelastic is sandwiched between the distal end of the threaded portion and the bottom portion of the recess portion. With this structure, the viscoelastic body is away from the circumferential wall by the length of the threaded portion. Therefore, this structure still has room for improvement in terms of increasing the vibration damping effect of the viscoelastic body. In the fixing structure disclosed in Japanese Patent Laid-Open No. 2005-160947, the viscoelastic body positions at a point higher than the circumferential wall of the sole portion by the length of the threaded portion. Therefore, this structure still has room for improvement in terms of lowering the center of gravity.